In this form of asexual reproduction, new polyps bud off from parent polyps to expand or begin new colonies. [a] This occurs when the parent polyp reaches a certain size and divides. This process continues throughout the animal's life and produces polyps that are genetically identical to the parent polyp.[b]
This method also allows a portion of an entire colony to establish a new colony. For instance, if a portion of a larger colony is broken off from the main colony during a storm or boat grounding, the separated individuals can start new coral colonies that are genetically identical to the parent colony. However, the sucess of the fragments in establishing a new colony is dependent upon whether they are exposed to favorable growth conditions. For instance, fragments exposed to strong wave action will find it difficult to settle on a substrate for continued growth.
To increase diversity in the gene pool, corals also reproduce sexually.
About three-quarters of all stony coral species are broadcast spawners and produce male and/or female gametes that are released into the water column in massive numbers, enabling them to distribute their offspring over a broad geographic area.[c] The gametes are positively buoyant and float towards the surface before the eggs and sperm join to form free-floating larvae called planulae. An individual planula floats in the water column until it finds a suitable space to call home - usually a hard surface to which it can attach.[c] Planulae are generally small enough that they can only be seen with a microscope. Large numbers of planulae are produced to compensate for the many hazards, such as predators, that they encounter as they are carried by water currents. The time between planula formation and settlement is a period of exceptionally high mortality for coral larvae.[b]
Along many reefs, coral spawning occurs as a synchronized event, when many coral species in an area release their eggs and sperm at about the same time. The timing of a broadcast spawning event is very important because male and female corals cannot move to make reproductive contact with each other. Because colonies may be separated by wide distances, the release of sperm and eggs must be precisely timed, and usually occurs in response to multiple environmental cues. There are both long-term and short-term controls that affect the timing of spawning events. The long-term control of spawning may be related to temperature, day length, or rate of temperature change. The short-term control is usually based on lunar cues. The final release, or spawn, is usually based on the time of sunset. [c] Broadcast spawning coral species may spawn on only one or a few nights each year, and though different species may spawn at different times, the spawning events for any given species happen at the same time.[d]
Whereas three quarters of stony corals are broadcast spawners, the remaining quarter of coral species are brooders. Brooding species generally have high success in recruiting new larvae into established colonies, but many of these species reach only small colony size and thus do not contribute much to the overall growth of a reef. In this reproduction mode, only male gametes are released into the water column. The male gametes are negatively buoyant and are transported by waves and current before sinking to the ocean floor. If encountered, the male gametes are then taken in by female coral polyps containing egg cells. Fertilization occurs inside the female coral and produces a small planula. This planula is released later through the mouth of the female coral at an advanced stage of development so that it is capable of settling onto hard substrate very soon after its release. Thus, brooding species generally disperse their larvae shorter distances from the mother colony than broadcasters. [d], [e]